The present invention generally relates to an auto-tuned apparatus, for a band-pass filter, for automatically setting a center frequency of a band-pass filter capable of changing the center frequency in accordance with the center frequency (hereinafter referred to simply as center frequency) of the signal passing band.
FIG. 15 is a block diagram of an automatic tuning type band-pass filter according to a first conventional embodiment proposed in Japanese Laid-Open Patent Application Tokaihei No. 1-105601.
In FIG. 15, the automatic tuning type band-pass filter in the first conventional embodiment comprises an isolator 101 which is adapted to pass in one direction high frequency signals to be inputted to the filter and is provided with a reflected wave coupling terminal 111, a resonator 102 operating as a band-pass filter for bandpass-filtering high frequency signals passing through the isolator 101, a drive mechanism 103 for changing the resonance frequency of the resonator 102 by the movement of a resonance frequency adjusting element (not shown) within the above described resonator 102, and a control circuit 104 for detecting with a diode D1 the high frequency signals which are outputted from the reflected wave coupling terminal 111 of the isolator 101, so as to control the drive mechanism 103 in accordance with the detected signals.
In the automatic tuning type band-pass filter, the level of the high frequency signals (hereinafter referred to as reflected signals) of the reflection power detected by the above described diode D1 is at a minimum at the resonance frequency of the above described resonator when a high frequency signal has been passed into the band-pass filter. By the use of the minimum level caused thereby, the above described control circuit 104 controls the driving mechanism 103 so as to maintain the level of the above described reflection signals at their minimum. The center frequency of the band-pass filter, which is approximately equal to the resonance frequency of the resonator 102, may thus be tuned to the frequency of the high frequency signal passing through the isolator 101.
The above described tuning operation is effected by the use of the fact that the level of the reflection signal is at a minimum at the resonance frequency of the resonator 102 in the automatic tuning type band-pass filter in the first conventional embodiment. Therefore, the problem exists that the above described tuning operation cannot be correctly effected when signals to be reflected from another channel are inputted into the automatic tuning type band-pass filter or when interference signals coming closer to the high frequency signals passing through the above described isolator 101 are inputted into the automatic tuning type band-pass filter if, for example, the automatic tuning type band-pass filter of FIG. 15 is used in a transmitter multiplexer.
The above described automatic tuning type band-pass filter is provided with one resonator 102. A multi-pole parallel connected band pass filter (hereinafter referred to as a second conventional embodiment) is disclosed in Japanese Laid-Open Patent Application Tokaihei No. 3-72701. In the multi-pole parallel connected band-pass filter, a plurality of resonators which are different respectively from each other, and have adjacent resonance frequencies, are electrically connected in parallel between the input terminal which receives the signal and the output terminal where the signal is outputted.
In order to adjust the center frequency and the band width of the multi-pole parallel connected band-pass filter in the second conventional embodiment, a network analyzer is connected to the input and output terminals of the multi-pole parallel connected band-pass filter. As sweep signals which sweep in frequency through the passband of the band-pass filter are inputted into the input terminal, the center frequencies of the individual band-pass filters are adjusted while the spectrum of the signals of the output terminal is being observed. The center frequency and the band width of the multi-pole parallel connected band-pass filter cannot be automatically adjusted, which leads to the problem that the adjustment is required to be manually effected.